Piezoelectric actuators are used in the automotive industry above all as final control elements for metering devices, consequently in particular for injection valves. They enables extremely fast switching of the control valve encompassed by the injection valve and thus allow, for example, a plurality of injection operations during a working cycle of a cylinder of an internal combustion engine. Piezoelectric actuating elements are also used increasingly in other sectors of the automotive industry, e.g. in braking systems.
Today's state-of-the-art engine management systems for combustion engines aim to provide operation producing the lowest possible exhaust emissions and fuel consumption, as well as to comply with (future) statutory framework regulations. The control system behavior of combustion engines is to be characterized as nonlinear owing to the dynamics of gasoline exchange and combustion and exhibits a marked temperature dependence. The mixture forming processes are influenced essentially by gasoline exchange, injection quantity, and injection profile. A crucial task is handled in this case by the precise control of the injection components. In this case injection quantity precision and injection time instant are elementary parameters for an optimal configuration of the combustion profiles. Modern injection elements, such as are used e.g. in common-rail or pump-injector systems, are already operated today with the aid of piezoelectric-controlled control valves.
One advantage of the piezoelectric-controlled control valves is, inter alia, that they not only act as an actuator, but by exploiting the piezoelectric effect can also be used as sensors. Information concerning deflection and acting forces, attrition or idle stroke can be generated in the process from the inherent sensor-like properties of said piezoelectric elements. Based on said information acquisition it is possible to construct regulation and control concepts which can precisely regulate internal system pressures and valve needle strokes with the aid of the energy balance of the actuating element.
However, said actuating elements also have disadvantages which are inherent in the nonlinear and hysteresis-affected behavior of the piezoelectric element itself. Contrary to the usual assumption, the material coefficients defining the behavior of the piezoelectric element are not independent constants, since said known ‘material constants’ describe the material properties only under small-signal conditions. In fact, however, their size varies with temperature, pressure and other boundary conditions, such that the behavior of complex components, such as piezoelectric-controlled injection valves, for which precise control is particularly important, cannot be adequately predicted under real-world usage conditions by means of a linear approach, in particular the assumption of a constant piezoelectric module dij. In the case of temperature and load changes the piezoelectric actuators have e.g. different coefficients of expansion (piezoelectric module) which in turn can be regulated only to a limited degree over all operating points and useful life by means of currently used linear regulation concepts and in view of the, for technical reasons, very high manufacturing tolerances of piezoelectric actuators.
In order to determine the material coefficients of piezoelectric elements, methods are known in the prior art which are resorted to on the one hand for small-signal measurements in which, as a consequence, only a comparatively low electrical energy (e.g. a few V/mm) is applied to the piezoelectric element. Known in particular in this context are impedance spectroscopy measurement methods with the aid of which it is fundamentally possible to determine the full data set of the elastic, dielectric and piezoelectric material parameters. Toward that end the piezoceramic sample is subjected to a sinusoidal alternating field in a predefined frequency range and its impedance is recorded as a function of the frequency by means of a commercially available impedance analyzer. In this case the resonance frequencies of piezoelectric test specimens of different geometries are measured. Through the combination of different oscillation modes it is possible to determine the complete data set of the piezoelectric element. Even if specific material coefficients are already predefined from other measurements and only one sample is measured in detail, said method is a relatively complex and cost-intensive laboratory method. The required investment in hardware for the computing power to be used therein appears not to be justifiable in practice outside of laboratory tests.
For measurements in the large-signal operating mode, on the other hand, the deflection and force of the piezoelectric element which occur during the activation by means of a corresponding field strength are currently measured directly. Highly complex measurements of this kind, in which the paths are very small and the forces very large, cannot be carried out without major difficulties during the operation of a piezoelectrically driven injection valve.
DE 198 04 196 A1 discloses a method for diagnosing a piezoelectric-mechanical system of an injection system currently in operation, wherein use is made of the fact that the state of the mechanical subsystem (to which the mechanical environment of the piezoelectric element also belongs) influences the electrical parameters of the piezoelectric element in a characteristic way, with the result that it is possible to detect malfunctions in the mechanical operation of the piezoelectric-mechanical system by recording and evaluating said electrical parameters. It is also known in this case to drive the piezoelectric element by means of a voltage signal of time-variable frequency and then to determine resonance frequencies from the extreme values of the electrical impedance profile. A frequency generator is provided in addition to a current and/or voltage detector for the purpose of generating the sinusoidal voltage signals.